Timer / Oscillator
555 Timer Calculator
Calculate R1, R2, and C for 555 timer circuits in astable (oscillator / PWM) or monostable (one-shot / pulse) mode. Results snap to standard E24 resistor values.
Select Mode
Astable generates a continuous square wave. Monostable outputs a single timed pulse when triggered.
Hz — e.g. 1000 for 1 kHz
% — must be > 50%
µF — e.g. 0.1 µF
milliseconds (ms)
µF — e.g. 10 µF
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555 Timer Formulas
Astable Mode (continuous oscillation):
f = 1.44 / ((R1 + 2×R2) × C)
Duty Cycle = (R1 + R2) / (R1 + 2×R2) × 100%
T_high = 0.693 × (R1 + R2) × C
T_low = 0.693 × R2 × C
Note: Duty cycle must be > 50% in standard config
Monostable Mode (single pulse):
T_pulse = 1.1 × R × C
Pulse triggered by LOW on pin 2 (TRIG)
Typical Component Ranges:
R1, R2: 1 kΩ – 1 MΩ · C: 100 pF – 1000 µF
Supply: 5V – 15V · Max output current: 200 mA
f = 1.44 / ((R1 + 2×R2) × C)
Duty Cycle = (R1 + R2) / (R1 + 2×R2) × 100%
T_high = 0.693 × (R1 + R2) × C
T_low = 0.693 × R2 × C
Note: Duty cycle must be > 50% in standard config
Monostable Mode (single pulse):
T_pulse = 1.1 × R × C
Pulse triggered by LOW on pin 2 (TRIG)
Typical Component Ranges:
R1, R2: 1 kΩ – 1 MΩ · C: 100 pF – 1000 µF
Supply: 5V – 15V · Max output current: 200 mA